1. Technical Field
The present invention generally relates to a light emitting device and, more particularly, to a structure and a method for manufacturing electrodes for such light emitting devices.
2. Description of the Related Art
Many types of light emitting elements are used in a wide range of devices such as light emitting diode (LED) displays and vehicle indicators. In particular, gallium nitride-based compound semiconductors such as GaN, InGaN, GaAlN have attracted a great deal of attention for use as materials for light emitting elements, LEDs, laser diodes (LDs), and the like which emit blue light. In the past, it was difficult to manufacture stable blue light emitting elements. However, these gallium nitride-based compound semiconductor materials make it possible to realize blue or green light emitting elements which have stability and strong intensity.
Generally, blue light emitting elements using a gallium nitride-based compound semiconductor are manufactured by growing a gallium nitride-based compound semiconductor layer such as GaN on a sapphire substrate. Since sapphire is an insulator, the electrodes of these elements cannot be formed on the substrate side. Therefore, the electrodes must be located on the compound semiconductor layer side. However, the electrodes block the light emitted from the compound semiconductor layers. Thus, the emitted light is taken out from the substrate side since the sapphire substrate is transparent. When the light emitting element is mounted on a lead frame, the electrodes on the compound semiconductor layer side must contact the lead frame.
The conventional structure of the light emitting elements using the gallium nitride-based compound semiconductor will be described below. FIG. 11 is a cross-sectional view of a conventional blue light emitting element. An N-type GaN buffer layer 3, an N-type GaN layer 5 and a P-type GaN layer 7 are grown on a sapphire substrate 1. P-type GaN layer 7 is partially removed by a conventional etching method to expose a portion of N-type GaN layer 5. An anode 17 is formed directly on the remaining portion of P-type GaN layer 7 and a cathode 9 is formed on the exposed portion of N-type GaN layer 5. Anode 17 and cathode 9 are connected to lead frames 53 by conductive (e.g., silver) paste layers 55a and 55b, respectively. In this light emitting element, light is emitted from the boundary between N-type GaN layer 5 and P-type GaN layer 7 when electrons and holes recombine. The light is reflected by anode 17 and is emitted through sapphire substrate 1.
It is difficult to reduce the size of the light emitting element of FIG. 11 because anode 17 and cathode 9 must be spaced apart sufficiently so that conductive paste layers 55a and 55b do not contact each other and so that N-type GaN layer 5 and P-type GaN layer 7 are not electrically connected together by conductive paste layer 55b at the region indicated by reference number 59 in FIG. 11. This means that the number of chips on a single wafer cannot increased and that the cost of the device can not be reduced. Further, the connections between the lead frames 53 and anode 17 and cathode 9 require high precision when the size is reduced. This inhibits the production of a large quantity of devices.
FIG. 12 is a cross-sectional view of a conventional GaAs-based compound semiconductor. An N-type GaAs buffer layer 33, an N-type AlGaInP cladding layer 35, an AlGaInP active layer 37, a P-type AlGaInP cladding layer 39, a P-type AlGaAs current spreading layer 57 and a P-type GaAs contact layer 42 are successively formed on an N-type GaAs substrate 31. One electrode 49 is formed on P-type GaAs contact layer 42 and the other electrode 51 is formed on the back side of N-type GaAs substrate 31. In this structure, injected current is spread in P-type AlGaAs current spreading layer 57. Light is emitted from the pn junction between AlGaInP active layer 37 and P-type AlGaInP cladding layer 39 and is output via current spreading layer 57.
Current spreading layer 57 must have considerable thickness. When current spreading layer 57 too thin, the injected current is not spread sufficiently and the light can not be emitted uniformly from the pn junction. In addition, current passes under electrode 49 and thus light is emitted from the portion of the pn junction under electrode 49. However, this light is blocked by electrode 49 and thus this light emitting device has a lowered light intensity.
Japanese Laid Open 7-131070 discloses an electrode structure for a light emitting device. The disclosed device is LED array type and has deep grooves around each light emitting region to isolate the pn junctions. The electrode of each light emitting region has a metal layer and a transparent conductive layer so that the electrode is able to form an ohmic contact to compound semiconductors. This electrode has a high transmittance for the wavelength of the emitted light. However, this structure does not effectively protect the pn junctions from the mesa etching to form the deep grooves. Therefore, the device has a lowered quality.